Highly accurate transition frequencies in the H2 Lyman and Werner absorption bands

The transition frequencies of low-J excitation lines in the B1Σu+– X1Σg+,(ν′,0) Lyman bands have been measured for ν′ = 2–18 and in the C1Πu–X1Σg+,(ν′,0) Werner bands for ν′ = 0–4, using a narrow-band tunable extreme UV laser source at wavelengths λ = 92–105 nm in conjunction with the technique of 1 + 1′ two-photon ionization. The measurements can be divided into three categories for which varying absolute accuracies were obtained. Special focus was on the B, ν′ = 2–5 bands, where an accuracy of 0.004 cm–1 or δv/v = 4 x 10–8 is achieved. For transitions to B, ν′ ≤ 13 and C, ν′ ≤ 3 states the accuracy is 0.005 cm–1 or δv/v = 5 x 10–8. Due to a different frequency mixing scheme uncertainties for B, ν′ ≥ 13 and C, ν′ = 4 are at the level of 0.011 cm–1 or δv/v = 1.1 x 10–7. Inspection of combination differences between R(J) and P(J + 2) lines shows that the accuracies are even better than estimated in the error budget. Based on the measurements of 138 spectral lines and the known combination differences, transition frequencies of 60 P-lines could be calculated as well, so that a data base of 198 accurately calibrated lines results for the Lyman and Werner bands of H2. Key words: vacuum UV, molecular spectroscopy, hydrogen, precision metrology.